Four-wheel and all-wheel drive vehicles have increased in use and popularity within the past few years. There have been many types of power transfer systems that are utilized for distributing torque power within these all-wheel drive and four-wheel drive motor vehicles in a full-time or part-time mode. Generally, most known four-wheel drive power transfer systems include a torque transfer device, such as a transfer case having an input shaft driven by a transmission output shaft, a rear output shaft driven by the input shaft and which is interconnected via a rear propeller shaft to a rear axle assembly for driving the rear wheels. A front output shaft interconnected to a front propeller shaft or front differential generally drives the front wheels. A torque transfer arrangement for continuously or selectively transferring drive torque from the rear output shaft to the front output shaft or from the front output shaft to the rear output shaft generally also is integrated therein. This interaxle differentiation of torque enables the front wheels and rear wheels to rotate at different speeds, which occurs during normal turning of the motor vehicle or when the vehicle is off-road on mud, loose gravel, ice, snow, water and the like.
Generally, in part-time four-wheel drive systems the transfer case is equipped with a shift mechanism which permits a vehicle operator to selectively couple and decouple the front and rear output shafts for shifting the vehicle between a two-wheel drive mode and a four-wheel drive mode. Full-time four-wheel drive systems have a transfer case that is equipped with an interaxle differential for continuously dividing drive torque between the front and rear output shaft while permitting speed differentiation therebetween. To prevent traction loss due to excessive wheel slip, many of these full-time transfer cases are equipped with a slip limiting device for selectively or automatically locking the interaxle differential to limit or prevent speed differentiation in response to wheel slip.
Recently there has been an increase in on-demand power transfer systems that are used for automatically directing power to the non-driven wheels without any input or action on the part of the vehicle operator but, only if traction is lost at the driven wheels. Typically, these speed sensitive torque transfer devices are installed between the front and rear output shafts for progressively delivering torque to the front output shaft in response to increasing speed differentiation therebetween. These torque transfer devices may commonly include viscous couplings, gear couplings, power couplings, electric couplings and the like.
Generally, transfer cases are generally classified as either a single offset or double offset type. In single offset transfer cases, only one of the output shafts is offset from the rotational axis of the input shaft. In double offset transfer cases, the front and rear output shafts are commonly aligned and are both offset from the rotary axis of the input shaft. One known disadvantage of double offset transfer cases is an increased underbody space that often creates packaging issues, particularly with off-road vehicles.
Another known problem associated with prior art transfer cases is the departure angles at connections between the transfer case front and rear output shafts and their corresponding propeller shafts. The departure angles are defined as the included angle between the rotary axis of the propeller shaft and that of the transfer case output shaft. Generally, in the prior art, single Cardan joints were used at each end of the propeller shafts if the departure angle was approximately less than 5°. If the departure angle exceeded 5°, then double Cardan universal joints or other additional components were required by prior art, necessitating an increase in cost and causing additional packaging concerns.
To reduce costs and minimize packaging concerns, there exists a need for a torque transfer device, such as a transfer case, that is capable of operating at the high departure angles found at the output shafts of the transfer case of modern-day four-wheel drive vehicles. These high output angles are anywhere from 10 to 20° in modern-day off-road or four-wheel drive vehicles. There also is a need in the art to produce more efficient packaging and reduction of extraneous noise in the transfer case caused by the use of constant velocity ball joints. Further, there is a need in the art for a more efficient way to transfer torque within the transfer case from the input shaft to both the rear output shaft and the front output shaft of the transfer cases.